1. Field of the Invention
The present invention relates to methods and apparatuses for reducing drag, and more particularly to methods and apparatuses for reducing drag produced by relative air movement on a moving body.
2. Description of Prior Art
Numerous means have been sought to improve the fuel-efficiency of moving bodies, and especially moving bluff bodies, by reducing their aerodynamic drag. In the field of surface transportation, and particularly in the long-haul trucking industry, even small improvements in fuel efficiency can reduce annual operating costs significantly. Previous investigations of aerodynamic drag of tractor-trailer trucks resulted in widespread adoption of air deflectors mounted on tractor cabs, and wholly redesigned tractors that utilize aerodynamic fairings to gradually increase the relatively small frontal area of tractors to match, and to blend smoothly with, the larger cross-section of typical trailers. Current air deflectors and fairings help guide the slipstream around the front of tractor-trailer trucks, and thereby reduce aerodynamic drag and improve fuel efficiency.
However, bluff bodies include generally flat rear ends, and the flat rear end of bluff bodies such as trailers is known to contribute significantly to aerodynamic drag. Current bluff bodies suffer from a severe pressure gradient from their widest point to their rear, such that a boundary layer therearound becomes stalled very rapidly, near the widest point. A stalled boundary layer causes flow to separate and a broad eddying wake to form downstream of the separation. The net result is the creation of considerable aerodynamic drag.
Previous attempts to streamline moving bodies such as bluff bodies have been constrained by legal and practical considerations. Federal and state regulations restrict the size of highway transport vehicles and inhibit conventional methods of streamlining because they limit the length and width of "add-on" aerodynamic devices. Current legal restrictions exclude devices that improve energy efficiency even if they result in only minor changes in overall dimension. Conventional streamlined afterbodies or "boat-tails" attached to the rear of trailers suffer the disadvantages of significantly increasing physical dimensions of the trailer and of interfering with loading and unloading. Additionally, when not in use, large retrofit "boat-tail" devices require additional space either for storage or for trailer parking, and further, limit the number of trailers that can be loaded onto flat-bed rail cars. Given the choice, trailer manufacturers currently prefer to design for maximum cargo capacity instead of minimum aerodynamic drag.
Therefore, a practical device is needed to reduce aerodynamic drag, particularly base pressure drag, from bluff bodies like tractor-trailer trucks. Such a device or apparatus needs to be in compliance with existing regulations, i.e., it needs to be within the scope of exclusionary clauses that permit only minor changes in the overall dimensions of tractor-trailer trucks. Further, the device or apparatus needs to reduce drag without interfering with cargo capacity, without altering current methods of loading and unloading, without requiring additional parking or storage space, and without changing current shipping practice.
Therefore, the present invention provides a method and apparatus for reducing the drag produced by relative air movement on a moving body, and especially a moving square-bodied bluff body. The present invention provides a method and apparatus for improving the fuel efficiency of a moving body, without significantly altering overall physical dimensions, without reducing cargo capacity, without significantly interfering with loading and unloading, without requiring substantially greater parking space, and without altering current shipping practices.